A variety of optical techniques has been proposed for detecting the end point of etching process. Observing changes in reflectivity of the layer being etched is described in U.S. Pat. No. 4,198,261. A technique using interference effects is described in Chapter 2, page 213, of Fine Line Lithography, North Holland Publishing Company (1980). These methods and other optical methods share the common requirement of directing the light used for the measurement onto the layer being processed. In modern semiconductor processing that layer can be represented by a very small feature size. Difficulty is then encountered in focusing the analyzing beam to the size of the feature and/or addressing that feature. A possible solution is to use a beam with an aperture larger than the feature. In that case, a change attributable to that feature will be detectable although the optical effect being measured will be masked to the extent of the portion of the beam that is incident on a feature or features where no optical change is occurring will exhibit no change. More severe problems arise when different parts of the region on which the beam is incident are changing in different ways e.g. where the process is an etching process and the beam is incident on portions having different etch rates. The following description is focused on this exemplary case. However, other applications can be envisioned which will make advantageous use of the teachings described here. In like vein, this description relates to differential etch rates between exposed and unexposed photoresist material. This is but one example of materials having different properties, which can be either physical or chemical, that cause them to behave differently under a given set of conditions.
Recognizing that this is exemplary the following background is relevant.
Techniques for detection of the completion of thin layer removal largely have been visual and empirical. An optical technique using the interference of reflected light for detecting the photoresist development end-point is described in the reference noted above.
However, as line width requirements for semiconductor integrated circuit fabrication become more stringent, the need for more precise photoresist masks increases. More accurate determination of end-point development is essential to avoid overetching and consequent loss of definition.
Accordingly, an embodiment of this invention is a more accurate method of determining the end-point in the development of photoresist coatings. It is a more accurate end-point determination technique which is automatic and does not depend on operator judgment.
The detection of the removal of a thin layer during an etching process is commonly referred to as "end point" detection. Typically this defines the point in time at which the layer underlying the layer being etched first becomes exposed. This may or may not signal the end of the etching operation. Often, etching is continued beyond the point of first exposure to allow clearing of the entire area that is to be removed. In these cases the so-called "end point" is actually an indicator of an intermediate point in the etching operation. A consequence of this can be a more accurate etching process since detecting the progress of the etching process allows for anticipation of the conclusion of the process and there is no error due to continued etching while the process is being turned off as would be the case when the ultimate end of the process is detected.